Thank you for your post.
I am unable to reproduce this problem. Here is our setup:
FileMaker Pro 10 Advanced on an iMac 24" running OS X 10.5.7, and a Dell Dimension running Windows XP SP3. From both machines, I send via SMTP Server, where the outgoing is the same for both Mac and Windows XP. No authentication. I am sending to the same email address and the Subject and Message are both set to "Test". Both emails are sent successfully.
Do you have access to a mail server that does not require authentication? This may help shed light on the issue. Also, make sure the mail server is spelled identical. I once spent an hour on the phone with a customer only to discover characters were switched (stmp instead of smtp).
Thanks for your efforts - the problem was that our Win machines and our Macs have different DNS servers, which have (had, now) different addresses for the SMTP server. We made them agree, although I could have had the email script determine the operating system and use the appropriate address.